When a node receives a replSetMaintenance:true request, it
adds a maintenance mode task to a queue of tasks. If the queue of
tasks was empty and now is not, the node will transition to
RECOVERING state and begin to reject read requests. When a node
receives a replSetMaintenance:false request, it removes a
maintenance mode task from the queue (even if that task was
initiated by a different client). If the request empties
the maintenance mode task queue, the node will return to
SECONDARY state.